Chalcogenide-based materials and improved methods of making such materials

ABSTRACT

The present invention provides strategies for making high quality CIGS photoabsorbing materials from precursor films that incorporate a sub-stoichiometric amount of chalcogen(s). Chalcogen(s) are incorporated into the CIGS precursor film via co-sputtering with one or more other constituents of the precursor. Optional annealing also may be practiced to convert precursor into more desirable chalcopyrite crystalline form in event all or a portion of the precursor has another constitution. The resultant precursors generally are sub-stoichiometric with respect to chalcogen and have very poor electronic characteristics. The conversion of these precursors into CIGS photoabsorbing material via chalcogenizing treatment occurs with dramatically reduced interfacial void content. The resultant CIGS material displays excellent adhesion to other layers in the resultant photovoltaic devices. Ga migration also is dramatically reduced, and the resultant films have optimized Ga profiles in the top or bottom portion of the film that improve the quality of photovoltaic devices made using the films.

PRIORITY

The present application is a divisional application of U.S. Ser. No.13/047,190, filed Mar. 14, 2011, which claims priority under 35 U.S.C.§119(e) from U.S. Provisional patent application having Ser. No.61/314,840, filed on Mar. 17, 2010, by Gerbi et al. and titledCHALCOGENIDE-BASED MATERIALS AND IMPROVED METHODS OF MAKING SUCHMATERIALS, wherein the entirety of said patent applications areincorporated herein by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates to methods for making chalcogenide-basedphotoabsorbing materials as well as to photovoltaic devices thatincorporate these materials. More specifically, the present inventionrelates to methods for making chalcogenide-based photoabsorbingmaterials, desirably in the form of thin films as well as tophotovoltaic devices that incorporate these materials, in which aprecursor film incorporating a sub-stoichiometric amount of chalcogen isprepared and then converted to the desired photoabsorbing compositionvia a chalcogenization treatment.

BACKGROUND OF THE INVENTION

Both n-type chalcogenide materials and/or p-type chalcogenide materialshave photovoltaic functionality (also referred to herein photoabsorbingfunctionality). These materials absorb incident light and generate anelectric output when incorporated into a photovoltaic device.Consequently, these chalcogenide-based photoabsorbing materials havebeen used as the photovoltaic absorber region in functioningphotovoltaic devices.

Illustrative p-type chalcogenide materials often include sulfides,selenides, and/or tellurides of at least one or more of copper (Cu),indium (In), gallium (Ga), and/or aluminum (Al). Selenides and sulfidesare more common than tellurides. Although specific chalcogenidecompositions may be referred to by acronyms such as CIS, CISS, CIGS,CIGST, CIGSAT, and/or CIGSS compositions, or the like, the term “CIGS”shall hereinafter refer to all chalcogenide compositions and/or allprecursors thereof unless otherwise expressly noted.

Photoabsorbers based upon CIGS compositions offer several advantages. Asone advantage, these compositions have a very high cross-section forabsorbing incident light. This means that CIGS-based absorber layersthat are very thin can capture a very high percentage of incident light.For example, in many devices, CIGS-based absorber layers have athickness in the range of from about 1 μm to about 2 μm. These thinlayers allow devices incorporating these layers to be flexible. Thinlayers use less material reducing the cost of the photovoltaic devices.This is in contrast to crystalline silicon-based absorbers. Crystallinesilicon-based absorbers have a lower cross-section for light capture andgenerally must be much thicker to capture the same amount of incidentlight. Crystalline silicon-based absorbers tend to be rigid, notflexible.

Industry has invested and continues to invest considerable resources indeveloping this technology. Making stoichiometric, photoabsorbingcompositions with industrially scalable processes, however, continues tobe quite challenging. According to one proposed manufacturing technique,deposition methods are used in an initial stage to deposit and/orco-deposit element(s) in one or more layers to form precursor film(s).Chalcogen(s) conventionally are incorporated via chalcogenization intothe precursor at a later processing stage. Chalcogenization ofteninvolves a thermal treatment in order to both incorporate chalcogen intothe precursor and to crystallize the film to convert the film to thedesired photoabsorbing layer. Because chalcogenization occurs after theprecursors are at least partially formed, these processes can bereferred to as “post-chalcogenization” processes.

There are many serious challenges to overcome with this approach. As onechallenge, chalcogenizing a precursor tends to induce significant volumeexpansion of the film. This expansion can cause mechanical stresses thatreduce adhesion, induce stresses, and/or other problems. It would behighly desirable to be able to reduce volume expansion whenchalcogenizing precursor films.

Additionally, very large voids tend to form in large part at the bottomof the film adjacent the backside contact (e.g., a Mo layer in manyinstances) as a consequence of chalcogenizing the precursor. These largevoids tend to cause adhesion problems between the CIGS layer and thebackside contact layer. Electronic performance and service life also canbe seriously compromised. These large voids also can induce mechanicalstresses that lead to delamination, fractures, and the like. It remainsvery desirable to find a way to reduce or even eliminate total number,size, and even location of these voids in the finished CIGS film.

As another drawback, post-selenization processes tend to inducesignificant gallium migration in the film. Generally, substantialportions, and even substantially all, of the gallium in upper filmregions tend to migrate to the bottom of the film. The top of the filmmay become completely Ga depleted. In the worst case, full Gasegregation can result such that the bottom of the film incorporates Cu,Ga, and Se, while the top of the film includes only Cu, In, and Se. Thismay have negative repercussions for film adhesion. Even moreimportantly, if some Ga is not located within the region of the CIGSfilm where much of the incident light is absorbed (e.g., in at leastapproximately the top 300 nm), the bandgap increase due to Ga isessentially lost. Such Ga depletion clearly compromises the electronicperformance of the photovoltaic device. It remains very desirable tofind a way to reduce and even eliminate gallium migration, or toincorporate Ga only into the areas of the film that yield a benefit(e.g., within the top approximately 300 nm of CIGS film for bandgapenhancement, and within the back region of film for adhesion relatedvoid modification and/or minority carrier mirror benefits.)

Some attempts have been made to prepare precursor films that incorporatestoichiometric amounts of chalcogen. For example, the literaturedescribes attempts to form CIGS films using compound targets of copperselenides and/or indium gallium selenides. These efforts have not beenclearly demonstrated to be brought to practice for an industrial processdue to the difficulty in fabricating such sputtering targets (especiallyfor the indium selenide based materials) and the difficulty in obtaininggood process control from such targets due to low sputter rates andtarget degradation.

As another alternative, high-quality CIGS material has been formed bythermal evaporation of all desired elements, including metals,chalcogen(s), and optionally other(s) onto a substrate at a highsubstrate temperature such that the film reacts and crystallizes fullyduring growth. Unfortunately, this evaporation approach is not trulyscalable with regard to industrial applications, particularly inroll-to-roll processes.

In contrast to thermal evaporation techniques, sputtering techniquesoffer the potential to be a more scalable method better suited forindustrial application of CIGS materials. However, it is verychallenging to supply the needed amount of chalcogen during thesputtering of Cu, In, Ga, and/or Al to form high quality CIGS in onestep. As a consequence, CIGS films have been sputtered from one or moremetal targets in the presence of selenium and/or sulfur containing gasor vapor from an evaporated source. Using only a gas or vapor as achalcogen source during sputtering of other components typicallyrequires that enough gas be used to at least supply the desiredchalcogen content in the precursor film, often with an additionaloverpressure of chalcogen. Using so much chalcogen-containing gas orvapor tends to cause equipment degradation and Se buildup, targetpoisoning, instabilities in process control (resulting in compositionand rate hysteresis), the loss of In from the deposited film due tovolatile In_(x)Se_(y) compounds, lowered overall deposition rates, andthe damage of targets due to electrical arcing.

SUMMARY OF THE INVENTION

The present invention provides strategies for making high quality CIGSphotoabsorbing compositions from sputtered precursor film(s) thatincorporate chalcogen(s) with a sub-stoichiometric concentration, withthe option of localizing a sub-layer of this chalcogen-containing layerat a specific location within the precursor film(s). The conversion ofthese precursors into CIGS photoabsorbing material via a followingchalcogenizing treatment (also referred to as “post-chalcogenization,”including, e.g., “post-selenization” when Se is used) provides one ormore or all of the following benefits: clearly reduced overall void areaand size; altered void location within the film such that a majority ofvoids are not located at the CIGS-back contact interface; improvedadhesion; reduced Ga migration upon chalcogenization; improved Gaconcentrations in the top region of the post-selenized CIGS filmrelative to a purely metal precursor film that is post selenized;improved Ga concentrations in the bottom boundary region; and improvedfilm smoothness resulting in fewer device defects via the improvement inconformality of overlying photovoltaic device layers.

In one aspect, the present invention relates to a method of making achalcogen-containing photoabsorbing composition. A precursor of thechalcogen-containing photoabsorber is formed wherein the precursorincludes a sub-stoichiometric amount of at least one chalcogen. Theprecursor is subjected to a chalcogenization treatment. The presentinvention also encompasses photovoltaic devices made by this method.

In another aspect, the present invention relates to a photovoltaicdevice that comprises a chalcogen-containing photoabsorbing film havinga top surface and a bottom surface. The photoabsorbing film comprises avoid content of 0.5 to 50%. The film also includes from about 0.1 toabout 60 atomic percent Ga at least in a first boundary region proximalto the bottom surface of the photoabsorbing film and/or a secondboundary region proximal to the top surface of the photoabsorbing film,wherein the atomic percent Ga is based on the total composition of thecorresponding boundary region.

In another aspect the present invention relates to a precursor film of achalcogen-containing photoabsorbing material. The precursor comprisesfrom about 0.1 to about 60 atomic percent Ga selectively co-incorporatedinto one or more portions of the precursor with at least one chalcogenbased upon the total composition of said portion; and wherein theprecursor includes a sub-stoichiometric amount of a chalcogen based uponthe overall composition of the precursor film.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other advantages of the present invention, andthe manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of the embodiments of the invention taken in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing how to make a photovoltaic deviceaccording to a representative embodiment of the present invention inwhich a single layer CIGS precursor film is prepared via sputtering froma single target;

FIG. 2 is a schematic diagram showing how to make an alternativeembodiment of a CIGS photoabsorbing layer, wherein multiple confocaltargets are used to prepare a single layer CIGS precursor film; and

FIG. 3 is a schematic diagram showing how to make an alternativeembodiment of a CIGS photoabsorbing layer, wherein multiple confocaltargets are used in a sequence of steps to prepare a multilayer layerCIGS precursor film.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

The embodiments of the present invention described below are notintended to be exhaustive or to limit the invention to the precise formsdisclosed in the following detailed description. Rather the embodimentsare chosen and described so that others skilled in the art mayappreciate and understand the principles and practices of the presentinvention. All patents, pending patent applications, published patentapplications, and technical articles cited herein are incorporatedherein by reference in their respective entireties for all purposes.

The principles of the present invention are used to form high quality,chalcogenide-based photovoltaic materials for use in photovoltaicdevices. As an overview, the methodology of the invention includes atleast two main stages. In an initial reaction stage, a precursor of thechalcogen-containing photoabsorber is formed wherein the precursoroverall includes a sub-stoichiometric amount of at least one chalcogen,although particular portions of the film might include a stoichiometricamount of chalcogen or even an excess. In a further stage, the precursorfilm is subjected to a chalcogenization treatment to incorporateadditional chalcogen into the film and thereby provide the compositionwith the final chalcogen content. If needed, the film optionally alsocan be subjected to a crystallization treatment such as by annealing orthe like to convert the precursor to a more desirable tetragonalchalcopyrite crystalline form, to increase grain size, and/or the like.Crystallization (e.g., annealing) and chalcogenization can occurseparately, in overlapping fashion, and/or simultaneously. Conveniently,the precursor film can be subjected to a thermal treatment in thepresence of one or more chalcogens that accomplishes bothchalcogenization and crystallization.

The present invention is versatile and can be used in a wide variety ofprocesses. For instance, the present invention can be practiced in batchor continuous processes. The present invention can be practiced at anindustrial scale. These principles allow these materials to be formed ina manner that is compatible with continuous, roll-to-roll manufacturingstrategies.

Using a chalcogen-deficient precursor as an intermediate to makephotoelectronically active chalcogen-containing materials provides manyadvantages. Quite significantly, this approach dramatically reduces thedegree of Ga migration upon post-chalcogenization in those precursorembodiments that incorporate Ga. Surprisingly, when a precursor film isprepared by co-depositing ingredients including Ga and a chalcogeneither throughout the film or within a particular portion of the film,the present invention dramatically reduces Ga migration from suchregions. In one embodiment in which Ga and sub-stoichiometric Se wereco-deposited throughout a precursor film also including Cu and In, Ga,migration was observed to be negligible. The final photoelectronicallyactive CIGS material had a constant Ga profile throughout the film, asmeasured by scanning XTEM with EDS (energy dispersive X-ray duringcross-sectional scanning transmission electronic microscopy)measurements of the film. Samples for measurements of Ga profiles areprepared using a standard cross-sectional technique suitable to renderthe film electron transparent so that TEM can be performed, such asfocused ion beam (FIB) cross-sectioning.

The Ga retention in the near-surface of the film can also be verified byquantum efficiency measurements of the finished cell that yield thebandgap of the absorptive part of the CIGS film. Without wishing to bebound, a theory to explain this advantage can be suggested. It isbelieved that the highly reactive, sputtered chalcogen species bondswith, and/or helps other film ingredients bond with, the Ga to formGa-containing species that are relatively immobile and much less proneto migration during chalcogenization.

Consequently, the Ga distribution of the photoelectronically active CIGSfilm tends to very closely match the Se and Ga distribution of theprecursor. If Se is deposited with a particular distribution orselective placement through the depth of the precursor, and theprecursor contains either Ga throughout the film, or selective Ga wherethe Se is deposited, generally a substantially similar Ga distributionor placement similar to the precursor Se placement will be maintained inthe final film. The ability to so easily control Ga distribution likethis in films prepared by chalcogenization techniques is quiteremarkable and believed to be accomplished with unprecedented ease.Indeed, it is believed that providing a CIGS film including substantialGa content in a boundary region adjacent the top surface of the CIGSfilm made via post chalcogenization techniques (which is evidenced bythe CIGS film having a void distribution) has not been achieved.

As another significant advantage, using a chalcogen-deficient precursoras an intermediate in chalcogenization processes also yieldsphotoelectronically active CIGS material with more favorable voidcharacteristics. Voids generally tend to form in the film product as anartifact of subjecting the precursor to a post-chalcogenizationtreatment. Indeed, the presence of significant amount or size of voidsis an indication that a CIGS film was produced using apost-chalcogenization process. Consequently, voids still tend to form inmany modes of practice of the present invention when CIGS precursorfilms incorporating sub-stoichiometric chalcogen are subjected topost-chalcogenization. However, the character of the voids is muchdifferent than those formed using conventional methods. Conventionally,too much of the void content is unduly large, e.g., having a size thatis at least about 25%, even at least about 50%, and even about 80% ormore of the final film thickness. These large voids tend to predominateat the interface between the CIGS film and the underlying substrate,e.g., at the interface between the CIGS film and an underlying Mo layerin many instances. This large content of large interface voids clearlycontributes to significantly reduced adhesion between the CIGS and theback contact layer, rendering the material difficult to process intophotovoltaic modules and the like.

In contrast, the present invention tends to produce CIGS films in whichthe average void size is measurably less according to cross sectionalelectron microscopy analysis, where the cross section of the film beinganalyzed is polished using a polishing technique compatible withscanning electron microscopic analysis, such as either ex-situion-polishing or focused-ion-polishing (FIB processed). Also, ratherthan disrupting the backside interface, the voids are distributedprimarily within the CIGS film itself in a manner that allows the CIGSfilm to function as a high quality photoabsorber with good adhesion tothe back electrical contact layer, even when a significant number and/orsize of voids remains. Per cross sectional electron microscopy analysis,CIGS material is easily detectable directly adjacent an underlying Molayer, with voids primarily within the CIGS layer itself, instead oflocated near or at the interface between the CIGS layer and theunderlying Mo layer.

For purposes of the present invention, the void content of a CIGS layerof a working cell is determined from scanning electron microscopy (SEM)analysis of polished cross-sections of the CIGS layer. Three randomcross-sections of the active area of a cell are obtained. Because thepractice of the present invention provides a very uniform voiddistribution, three samples are sufficient to represent the film as awhole. The three samples may be taken from any part of the active area.Preferably, the samples are non-parallel. For instance, if the activearea of the cell is deemed to lie in the x-y plane, respectivecross-sections can be taken along the x axis, the y-axis, and at anangle, e.g., 45° between the x and y axes. Each sample desirably is longenough such that the total length of all the samples measured desirablyis at least 20 microns (for example, 10 samples each showing 2 micronsof cross section; 5 samples each showing 4 microns of cross section; 3samples each showing 7 microns of cross section; although it is notrequired that all samples have the same length). The thickness of eachsample is not critical and may be any thickness suitable for handlingand for mounting into the microscope instrument being used. One face ofeach sample is polished to aid microscopic analysis. Any polishingtechnique compatible with microscopic analysis may be used, as a varietyof polishing techniques provide comparable polishing results. Focusedion beam polishing and external ion beam polishing are representativetechniques. External ion beam polishing is preferred as being morewidely accessible.

SEM is used to evaluate the polished surface and determine void contentof the cross-section as a percentage area of the cross section occupiedby voids. Because SEM analysis is being used, only the one face beingevaluated needs to be polished. The void content of each individualcross-section is determined as 100%×A_(v)/A_(T), where A_(v) is the voidarea of the cross-section and A_(T) is the total area of thecross-section. Because A_(v)/A_(T) is a ratio, any units for area can beused so long as the same units are used for both A_(v) and A_(T). Thevoid content of the sample is the average of the three samples.

In illustrative embodiments, a CIGS photoabsorbing film made usingprinciples of the present invention may have a void content of at least0.5%, preferably at least 1%, more preferably at least 5%, morepreferably still at least 10% and most preferably in some embodiments atleast 20%. Desirably, the void content is no greater than 60%,preferably no greater than 50% of the cross-sectional area of the CIGSfilm when determined according to the methodology described herein. Suchvoids desirably have an average size in the range of at least about 0.1microns, more preferably at least 0.2 μm and no greater than 2 microns,preferably no greater than 1.5 microns and more preferably no greaterthan 1.2 μm when determined according to cross-sectional scanningelectron microscopy with samples prepared in cross section using thefocused ion beam technique. For non-circular voids or aggregated voidareas, the void size is deemed to be the length of the greatest chordthat extends across the void(s). Desirably, at least more than 10%, morefavorably 30%, and more favorably 50%, of the total void area includesvoids that contact inner surfaces of the CIGS material only, and do notcontact the CIGS-Mo interface.

As another advantage of using precursors with stoichiometricallydeficient chalcogen content, volume expansion can be significantlyreduced when the precursor is subjected to a chalcogenization treatment,depending on the amount and location of chalcogen in the precursor film.Reduced volume expansion is reflected in the reduction of total voidcontent. It is noted, however, that volume expansion ma_(y) not be theonly mechanism that causes formation of voids. Without wishing to bebound, another cause could be attributable to the substantialsegregation of metals within the precursor metal film whether depositedas separate layers or as an alloy. At the metallic ratios commonly used(e.g. at least (In+Ga)/Cu=1 or higher), indium may tend to exist assegregated islands on the surface of a CIG alloy film (when sputteringfrom a CIG alloy target, or CuGa+In confocal targets), or metallic Cuand In could exist if depositing the metals separately as layers. Thesemetals react at different rates upon post-selenization in a very complexfashion. The use of a more isotropic precursor film according to thepresent invention may help to avoid undesirable phases duringpost-selenization, and/or reduce unwanted diffusion or melting ofsegregated metallic areas.

Chalcogenizing a stoichiometrically deficient precursor also allows thechalcogenization step to be carried out uniformly and rapidly.Chalcogenizing a conventional, chalcogen-free precursor is less uniformin several respects. In a conventional process in which a chalcogen suchas Se is incorporated into the film only after a metallic precursor isformed, for instance, Se reaches some portions of the film right away.Other portions see the Se only after a period of delay. Consequently,some portions of the film are subjected to Se conditions longer thanother portions as the migration occurs. The extent and/or nature of thereactions that occur will vary with the migration as a consequence. Incontrast, when practicing the present invention, the chalcogen alreadypartially exists generally throughout the precursor film or in selectedareas. Selenization conditions are more uniform inasmuch as thedependence upon migration is substantially reduced. Additionally, theinitial precursor film is likely not a single alloy, but could be amixture of Cu, Ga, In, and alloys thereof. The precursor film mayundergo more complex reactions during post-chalcogenization andannealing, with undesirable phases forming. If chalcogenide is onlysupplied as a “cap” of solid material or a gas after the precursor filmis fully formed, chalcogen migration into the film could occur onlydirectionally via a top-down process leading to stress and/or undulylarge voids.

The fact that the stoichiometrically deficient precursor film yields ahigh quality, industrially compatible (e.g., capable of beingincorporated into continuous, high speed processes with large areadeposition such as continuous roll to roll processes), CIGSphotoabsorbing layer upon chalcogenization is quite surprising. From aconventional perspective, sputtering from sources including chalcogengenerally yields an extremely poor quality film precursor. As oneserious deficiency, the initial sputtering stage generally producesoverall a CIGS precursor film incorporating a sub-stoichiometric amountof chalcogenide, even when a stoichiometric CIGS target is used. Thedeficiency is exacerbated by the fact that chalcogen may react withother precursor constituents to form volatile species, e.g.,In_(x)Se_(y) species, which can be volatized and lost.

Additionally, although sputtered stoichiometric CIGS films have beenused in attempts to make high quality CIGS photoabsorbers, thesub-stoichiometric CIGS compositions have not. Chalcogen vacancies insubstoichiometric films create very undesirable electronic defects inCIGS thin films, typically rendering the material incapable of forming aworking photovoltaic device. Efficiencies are very low and even zero inmany instances. Substoichiometric CIGS compositions also may suffer fromother serious deficiencies such as reduced light absorption, poorcarrier mobility and lifetime (likely due to the chalcogen vacancies),and poor grain structure. These deficiencies may also be related to anon-optimized post-selenization reaction, which concurrently anneals thefilm. Accordingly, the industry tends to disregard and discard thesesub-stoichiometric films as viable candidates for making high qualityCIGS photoabsorbing compositions.

Conventional wisdom would be strongly motivated to disregard and discardthe deficient product as a viable route to high quality CIGSphotoabsorber layers on the industrial scale. The present invention isquite significant because the present invention overcomes this strongnegative bias. The present invention is based on the discovery that,although a precursor film intentionally deficient in chalcogen does notitself form a good photoabsorber, such a film nonetheless is anexceptional intermediate and stepping stone on a path to produce highquality, photoabsorbing CIGS material. Without the “pre”-addition ofchalcogen into the precursor itself, these precursor films would havebeen viewed as sub-optimal inasmuch as they would exhibit very largevoid formation, too much volume expansion, subsequent poor adhesion,and/or substantially no Ga within the top portion of the film uponchalcogenization.

In many embodiments, the photoelectronically active chalcogenidematerials useful in the practice of the present invention preferablyincorporate at least one IB-IIIB-chalcogenide, such as selenides,sulfides, tellurides, and/or combinations of these that include at leastone of copper, indium, aluminum, and/or gallium. More typically at leasttwo or even at least three of Cu, In, Ga, and Al are present. Sulfidesand/or selenides are preferred. In many embodiments, these materials arepresent in polycrystalline form. Some embodiments include sulfidesand/or selenides of copper and indium. Additional embodiments includeselenides or sulfides of copper, indium, and gallium. Specific examplesinclude but are not limited to copper indium selenides, copper indiumgallium selenides, copper gallium selenides, copper indium sulfides,copper indium gallium sulfides, copper gallium selenides, copper indiumsulfide selenides, copper gallium sulfide selenides, and copper indiumgallium sulfide selenides (all of which are referred to herein as CIGS)materials. Such materials are referred to by acronyms such as CIS, CISS,CIGS, CIGST, CIGSAT, and/or CIGSS compositions, or the like(collectively CIGS compositions hereinafter unless otherwise expresslynoted otherwise). CIGS materials also may be doped with other materials,such as Na, Li, or the like, to enhance performance. In manyembodiments, CIGS materials have p-type characteristics.

Oxygen (O) is technically a chalcogen according to its placement in theperiodic table of the elements. However, oxygen is deemed not to be achalcogen for purposes of the present invention inasmuch as oxygen doesnot contribute to photoabsorbing functionality to the extent of theother chalcogens such as S and/or Se. Even though oxygen does notpromote photoabsorbing functionality to the same degree and/or in thesame manner as Se or S, oxygen may still be incorporated into CIGSmaterials, e.g., many chalcogen materials could incorporate at leastsome oxygen as an impurity that does not have significant deleteriouseffects upon electronic properties.

Advantageously, the chalcogen-containing, photoabsorbing materialsexhibit excellent cross-sections for light absorption that allowphotoelectronically active films incorporating these materials to bevery thin and flexible. In illustrative embodiments, a typical absorberregion may have a thickness in the range from about 0.8 μm to about 5μm, preferably about 1 μm to about 2 μm.

One preferred class of CIGS materials may be represented by the formula

Cu_(a)In_(b)Ga_(c)Al_(d)Se_(w)S_(x)Te_(y)Na_(z)  (A)

Wherein, if “a” is defined as 1, then:“(b+c+d)/a”=1 to 2.5, preferably 1.05 to 1.65“b” is 0 to 2, preferably 0.8 to 1.3“c” is 0 to 0.5, preferably 0.05 to 0.35d is 0 to 0.5, preferably 0.05 to 0.35, preferably d=0“(w+x+y)” is 1 to 3, preferably 2 to 2.8 (note, (w+x+y)<2 forsubstoichiometric precursor films)“w” is 0 or more, preferably at least 1 and more preferably at least 2to 3“x” is 0 to 3, preferably 0 to 0.5“y” is 0 to 3, preferably 0 to 0.5“z” is 0 to 0.5, preferably 0.005 to 0.02

The copper indium selenides/sulfides and copper indium galliumselenides/sulfides are preferred. Strictly stoichiometric illustrativeexamples of such photoelectronically active CIGS materials may berepresented by the formula

CuIn_((1-x))Ga_(x)Se_((2-y))S_(y)  (B)

where x is 0 to 1 and y is 0 to 2. As measured and processed, such filmsusually include additional In, Ga, Se, and/or S.

Corresponding precursors of such CIGS materials generally would includeconstituents in the same proportions as specified in Formula A or B,including additional In and/or Ga as applicable to compensate for Inloss during post-chacogenization, except that the chalcogen content issub-stoichiometric in the precursor. A stoichiometric CIGS compositiongenerally includes at least one chalcogen atom for each metal atomincluded in the composition. A CIGS composition is sub-stoichiometricwith respect to chalcogen when the ratio of chalcogen atoms to the sumof all metal atoms is less than 1. Thus, with regard to Formula A,w+x+y=a+b+c+d for a fully stoichiometric material. However for thesubstoichiometric precursor, the ratio of w+x+y to a+b+c+d is less than1, preferably less than about 0.9, more preferably less than about 0.8,even more preferably less than about 0.6 and even more preferably lessthan about 0.4. On the other hand, it also is preferred that sufficientchalcogen is incorporated into the precursor to provide a desiredbenefit. Accordingly, the ratio of the ratio of w+x+y to a+b+c+d in thesubstoichiometric precursor is at least about 0.01, more preferably atleast about 0.2, and more preferably at least about 0.3. These ratioscan be determined by ICP-OES (inductively coupled plasma opticalemission spectroscopy).

The precursor film is sub-stoichiometric with respect to chalcogenoverall. However, if chalcogen(s) are selectively incorporated into oneor more portions of the precursor, those portions independently mayinclude substoichiometric, stoichiometric, or even an excess ofchalcogen locally.

The ICP-OES method can be used to measure a chalcogen deficiencyindicative of a precursor that is sub-stoichiometric with respect tochalcogen. Using this technique, a film is stoichiometric with respectto chalcogen when the measured atomic ratio of Se/(Cu+In+Ga) is 1.0 orhigher. For sub-stoichiometric films, this ratio is less than 1.0. Forexample, one precursor film has a Se/(Cu+In+Ga) ratio of 0.88 as grown(Example 1 below); another has a Se/(Cu+In+Ga) ratio of 0.33 as grown(Example 2 below), with the Se distributed throughout the film; Example3 below another has a Se/(Cu+In+Ga) ratio of 0.07, with all of the Selocated in a thin layer approximately 0.1 microns deep at the back sideof the precursor film. Sub-stoichiometric characteristics aredistinguished from fully stoichiometric CIGS film that lose only aslight amount of Se at the top surface, such as if it is heated in avacuum without a Se source to cause Se vacancies. Such a film may stillhave a Se/(Cu+In+Ga) ratio of 1.0 or higher as measured by ICP.

The precursor films may include one or multiple layers. In precursorembodiments including two or more layers, the interfaces between layersmay be distinct or can be graded transitions. A graded transition is onewhere the interface between the layers is not sharp and easilyidentifiable. Precursor films with multiple layers allow theconstituents of the precursor to be independently and easily allocatedamong one or more of the layers. This allows constituents such aschalcogen(s), Ga, or others to be selectively placed in some layers butexcluded or used in lesser amounts in other layers. For example,chalcogen(s) may be incorporated (optionally co-incorporated with Ga orAl) into one or more selected regions of the precursor film in which thepresence of the chalcogen(s) would be most desirable. As one option,chalcogen(s) can be incorporated into a layer of the precursor film thatis proximal, preferably directly proximal, to the backside in order topromote adhesion, void reduction, and/or to help function as a minoritycarrier mirror. As another or additional option, chalcogen(s) can beincorporated into a layer of the precursor film that is proximal,preferably directly proximal, to the top of the precursor film tofacilitate Ga incorporation (via reduced Ga migration, for example) andcorresponding bandgap enhancement. Chalcogen(s) need not be incorporatedinto other portions of the precursor, or if present could beincorporated in lesser amounts.

For example, chalcogen-containing target(s), or a mixed sputtering andvapor or gas phase chalcogen process, could be used to deposit theselected regions designated to include chalcogen content. The rest ofthe film could be provided by standard Cu, In, CuGa, or CuIn, CIG alloytargets, and/or the like. Thus, in one embodiment, a CIGS precursor filmincludes at least a first region of the film that incorporateschalcogen, at least a second region that incorporates chalcogen, and athird region interposed between the first and second regions thatincludes substantially no chalcogen as deposited. This can be measuredwith the scanning XTEM with EDS technique across a cross-sectionalsample of the film.

When chalcogen is selectively placed into regions (each of whichindependently may be formed of single or multiple layers but is oftenformed from a single layer), each such region independently may have athickness selected from a wide range. In illustrative embodiments, eachsuch region independently has a thickness of at least about 5 nm,preferably at least about 50 nm, more preferably at least about 100 nm,and desirably the thickness is less than about 1000 nm, more preferablyless than about 500 nm, and more preferably less than about 400 nm. Inone embodiment a region having a thickness of about 300 nm would besuitable. Generally, individual layers used to make the overall filmwould independently have thicknesses in these ranges as well. In view ofthe risk of Ga migration, Ga desirably is selectively co-incorporatedinto the precursor with chalcogen(s). Optionally, Ga need not beincorporated into other regions of the precursor or could beincorporated in lesser amounts.

A precursor film formed from a single or multiple layers can have anoverall thickness over a wide range. Generally, if the overall film istoo thin, the layer may not be continuous after post-processing or mayyield a final film have a low cross-section for capturing incidentlight. Layers that are too thick are wasteful in that more materialwould be used than is needed for effective capture of incident light.Thicker layers also present more recombination defect opportunities forthe charge carriers, which could degrade cell efficiency. Balancingthese concerns, many illustrative embodiments of precursor filmsincluding at least some chalcogen have thicknesses of at least about 0.1microns, preferably at least about 0.4 microns, and preferably thethickness is up to about 0.6 microns, preferably up to about 2.0microns. In one mode of practice, a precursor film having a thickness ofabout 0.6 microns was converted to a final CIGS film having a thicknessof about 2 microns. Precursor films that include larger amounts ofchalcogen tend to be thicker than precursor films that contain lesseramounts of chalcogen.

Forming precursor films from multiple layers offers the opportunity toincrease throughput when sputtering techniques are used to make at leasta portion of the precursor. Generally, targets incorporatingchalcogen(s) such as Se sputter much more slowly than targets that arechalcogen-free. To increase overall production rates, sputtering canbegin according to an illustrative embodiment by sputtering fromtarget(s) incorporating Se, S, and/or Te to form a thinchalcogen-containing layer proximal to the substrate. For reasonsincluding reducing Ga migration, it is also desirable that a layer a_(t)the top of the precursor incorporates chalcogen(s) as well. One or moreadditional layers can be sputtered at faster rate(s) by using targetsthat are chalcogen-free. This approach provides the benefits of usingsputtered chalcogen while also accessing higher deposition rates for aportion of the deposition when chalcogen is not being sputtered.

Because Ga is expensive, it would be valuable to incorporate thiselement, if present, only into selected region(s) where the presence ofGa yields sufficient benefit. This also can be implemented easily inmultilayer precursors by incorporating Ga only into the appropriatelayers. In one mode of practice, Ga would be deposited into the topregion (which may be formed from one or more layers) of the precursorfilm that includes at least a portion, preferably at least substantiallyall of the film depth in which light is absorbed so that the bandgapexpansion of Ga helps to augment device performance. In manyillustrative embodiments, this corresponds to including Ga in the top ofthe film to a depth of at least about 10 nm, preferably at least about20 nm, more preferably at least about 30 nm up to a depth of at leastabout 1000 nm, preferably up to about 300 nm, more preferably up toabout 200 nm in the top boundary region of the precursor. It can also behelpful to include Ga in at least the bottom boundary region of theprecursor film to a depth of at least about 10 nm, preferably at leastabout 20 nm, more preferably at least about 30 rim up to a depth of atleast about 1000 nm, preferably up to about 300 nm, more preferably upto about 200 nm. In this bottom boundary region (which may be formedfrom single or multiple layers), the Ga may help with adhesion and/oract as a “minor” to minority carriers. Such Ga-containing regions may beformed from single or multiple layers.

Because the benefits of using Ga tend to be most prominent with respectto Ga included in the boundary regions, Ga preferably is present in bothboundary regions. Ga between these regions is optional and even can beomitted to save substantial expense in manufacturing costs. Thus, insome embodiments, at least one region may exist between the boundaryregions that includes substantially no Ga. To help reduce Ga migration,the regions including Ga desirably also include at least one chalcogen.Regions that do not include chalcogen may still include Ga, if desired.This Ga may migrate during post-selenization, but the migration of Gawhere the chalcogen was located will be reduced, forming the desired endprofile.

The amount of Ga included in one or both of these boundary regions canvary over a wide range. Desirably, preferred modes of practice wouldprovide sufficient Ga in the top region of the final CIGS film to asuitable depth to capture incident light such that the bandgap of theCIGS in this active area of the film is approximately 1.1 to 1.3 eV. Inmany representative embodiments, a CIGS precursor film is preparedwherein the film includes from about 0.1 atomic percent to about 60,preferably about 2 to about 10 atomic percent Ga in boundary regionsadjacent at least one of the top and bottom surfaces of the CIGS film.

A variety of methods can be used to form the precursor films. Examplesinclude sputtering of chalcogen-containing targets, optionally at thesame time as sputtering non-chalcogen containing targets; sputteringchalcogen-free targets in the presence of chalcogen-containing gas(es)or vapor(s); using evaporation techniques to provide all constituents;chemical ink rolling techniques; combinations of these, and the like.Sputtering techniques are preferred in which one or more targets areused to deposit the precursor film onto a substrate or portion of asubstrate, wherein at least one sputtering target incorporates at leastin part a chalcogen selected from Se, S, and/or Te. Se and/or S are morepreferred. Se is particularly preferred.

Sputtering offers many advantages. Sputtered chalcogen species (such asionized Se) are more reactive than Se compounds (such as but not limitedto Se₂ or Se₆ or Se₈ rings, etc.) that are formed by processes relyingsolely upon evaporation or other similar techniques. Via sputtering,reactive Se is very easily incorporated into the growing precursor film.While not wishing to be bound by theory, it is believed that thesputtered Se is in the form of highly reactive ion gas phase species,such as Set If the sputter process is performed at low enough pressuresand small enough distances between the target and substrate, the kineticenergy and ionization state of these precursor species is substantiallypreserved, further promoting incorporation into the depositing precursorfilm.

Generally, due to factors including the reactivity of the sputteredchalcogen, the sputtered species impinge upon the substrate and areincorporated into the growing precursor film at high efficiency. More ofthe chalcogen such as Se ends up in the precursor film rather thancoating and contaminating the chamber, as otherwise tends to occur outof place with processes such as hybrid sputtering/evaporation. Theoverall amount of Se needed is reduced since less is wasted.

As another advantage, sputtering need not occur in the presence ofadditional evaporated or other chalcogen source(s). This is highlydesirable inasmuch as Se- and S-containing gases such as H₂Se and H₂Sare costly and require careful handling. The use of these gassesreactively can poison metallic or metallic alloy sputter targets and cancause a significant loss of indium in the growing film.

Furthermore, lesser amounts of chalcogen material may be needed in thesubsequent chalcogenization (“post-selenization”) stage, since a portionof the desired chalcogen is already incorporated generally throughout orin selected areas of the precursor film.

Sputtering techniques also are easily applied to form precursor filmsthat are deficient in chalcogen. Making a precursor film with asub-stoichiometric amount of chalcogen is relatively easy in contrast tothe challenge of forming a high quality stoichiometric film viasputtering or reactive sputtering alone. One reason for this difficulty,as exemplified by sputtering Se containing targets, is that a portion ofthe Se is lost during the sputtering process. Using targets with excesschalcogenide is not a practical option to overcome the Se loss, as suchtargets are difficult to make due to the unsuitability to pure Se orhigh-Se binary phases for sputtering. Even if sputtering usesstoichiometric targets, a precursor that is sub-stoichiometric in Se mayresult anyway. Another reason for this difficulty is the positioning oftargets or vacuum machinery using reactive sputtering or hybridprocesses when a large amount of Se is needed.

Consequently, the present invention offers the opportunity to use one ormore targets in which the chalcogen content within the target(s) issub-stoichiometric. These targets may be much easier and less costly tomake. Additionally, the sputtering rate favorably tends to increase asthe chalcogen content in the target is lower. Consequently, usingtargets with sub-stoichiometric chalcogen content offers the opportunityfor faster throughput (such as Cu₂Se).

One or more targets can be used to form the precursor CIGS film so longas at least one target includes at least one chalcogen and the target(s)containing chalcogen(s) are used to form the precursor CIGS film for atleast a portion of the total sputtering time.

If more than one target is used, these can be used simultaneously, inoverlapping fashion, and/or in sequence. Different sputtering parameterscan be used with different targets to optimize depositioncharacteristics. In preferred embodiments, the chalcogen containingtargets may include sub-stoichiometric amounts, stoichiometric amounts,or even an excess of one or more chalcogens. One exemplary chalcogencontaining target includes copper and selenium, such as a Cu₂Se target.Another exemplary chalcogen containing target includes Cu, In, Ga, andSe. Chalcogen containing targets may be used singly or in combinationwith one or more other targets, such as metallic targets containing Cu,In, CuGa, CuIn, CIG, or the like.

One factor impacting target selection concerns whether the precursorfilm will be formed from single or multiple layers. A single layerprecursor film that incorporates Cu, In, Ga, and Se, for example, can beformed in one embodiment by sputtering a target containing all theseingredients onto the desired substrate. Multiple targets also may beused to form a single layer precursor film. As one example, a singlelayer precursor film that incorporates Cu, In, Ga, and Se can be formedaccording to an illustrative embodiment by confocal sputtering from afirst target containing Cu, In, Ga, and Se, and a second targetcomprising Cu, In, and Ga. Desirably, the second target is In-rich withrespect to the stoichiometric amount of In to help ensure that enough Inis incorporated into the precursor film.

Exemplary embodiments of targets suitable in the practice of the presentinvention are described further below in connection with FIGS. 1 through3. Suitable targets containing at least one chalcogen and at least oneof Ga, In, Cu, Na, Al, Li, O, combinations of these, and the like can besourced commercially.

In selecting sputtering parameters, one consideration is to avoid unduecollisions of the desired species from the point of creation to thearrival at the growing film at the substrate. Those knowledgeable in theart consider this to mean that the “mean free path” of the sputteredspecies desirable is greater than the target-to-substrate distance. Topractice such principles, exemplary modes of practice are characterizedby a target to substrate distance in the range from at least about 1 cm,preferably at least about 3 cm up to about 30 cm, more preferably up toabout 15 cm, with sputter pressures of at least about 0.5 mTorr,preferably at least about 3 mTorr up to about 50 mTorr, more preferablyup to about 3 to 5 mTorr. In one embodiment, a target-to-substratedistance of about 8 to 10 cm coupled with a sputter pressure of 3 to 5mTorr would be suitable.

Sputtering can be carried out at a wide range of substrate temperatures.As used in this context, the sputtering temperature refers to thesurface temperature of the substrate onto which the precursor film isbeing grown. For example, sputtering may occur at room temperature withno intentional substrate heating. On the other hand, the temperatureshould not be so high so as to unduly degrade constituents of thesubstrate or the growing precursor film. As general guidelines, usingtemperatures in the range from about −25° C. to about 650° C.,preferably about 5° C. to about 600° C., more preferably about 20° C. toabout 550° C., would be suitable in many embodiments. In exemplaryembodiments, sputtering temperatures initiated at about room temperaturewithout intentional heating other than that provided by the heat ofreaction (“room temperature”), 240° C., 400° C., and 550° C. allprovided precursor films that were converted into high quality CIGSphotoabsorbing compositions. Substrate measurements indicate that thesubstrate temperature initiated at about room temperature withoutintentional heating reached about 60° C. during growth. At thistemperature, the resultant chalcogen-containing precursor films appearto be amorphous and featureless by SEM, with a very slight, small grainsize (<100 nm) of CIGS material detected by XRD.

Sputtering temperature impacts the phase characteristics of theprecursor film. Without wishing to be bound, it is believed that someprecursor films may incorporate substantial amorphous content. XRDanalysis of precursors shows that crystalline content appears to becomegreater at higher sputtering temperatures. For example, a single layerCIGS precursor film incorporating Cu, In, Ga, and Se was sputtered atroom temperature. During the course of sputtering, the substratetemperature increased without additional heating to 60° C. SEM analysissuggests this precursor film has a predominantly featureless structurewith substantially no observable crystalline facets or grains, but smallcrystal grains on the order of about 11.6 nm in size are detected byXRD. The precursor film had an (In+Ga)/Cu ratio of 1.05 and a Se/Curatio of 2.04. Another embodiment of a precursor film that was sputteredat 400° C. using the same kind of target included slightly largercrystalline grains on the order of about 35.2 nm in size by XRD. Thisembodiment showed an (In+Ga)/Cu ratio of 1.08 and Se/Cu ratio of 1.82.An additional embodiment of a precursor film that was sputtered at 550°C. using the same kind of target and according to SEM and XRD analysishad a full, high quality crystal structure with crystalline grain sizegreater than 100 nm.

Consequently, sputtering temperature can impact the steps that might beused subsequently to convert the precursor to the final form.Temperature may also impact the parameters used to carry out thosesteps. For example, the precursor film tends to be more amorphous withsmaller crystalline domains at lower sputtering temperatures.Consequently, the precursor desirably is chalcogenized and crystallizedin order to obtain the desired tetragonal, chalcopyrite phase. If theprecursor film is formed above about 500° C., the precursor film mayalready be converted to the desired separate crystalline structure and acrystallizing step may not be required. In this case, chalcogenizationmay be all that is required to complete the CIGS layer, andchalcogenization may occur at lower temperatures, in less time, and/orunder different conditions. Even in such embodiments in which sputteringoccurs above about 500° C., annealing may still be desirable to furtherimprove crystalline characteristics if desired.

Analysis of a sample where the substrate was maintained at about 400° C.during sputtering also shows how Ga migration advantageously is reducedafter chalcogenization when practicing the present invention. Analysisof the precursor film showed that the sample had a very even Ga profilethrough film depth as deposited, and that this profile remained evenafter being converted to a CIGS photoabsorber. The resultant CIGSphotoabsorbing layer also was characterized by very smooth interfaces,reduced void content, and voids reduced in size. The layers werefavorably quite dense. In contrast, a conventional process (in which aprecursor film is formed from sputtered films in the absence ofsputtered chalcogen that are later selenized) yields CIGS layers withextremely rough surfaces, poor adhesion, flaky composition, many hugevoids (on the order of 50% to 100% of the film thickness itself), and anear total Ga deficiency near the top surface due to severe Gamigration. The selenized sample films also showed excellent adhesioncharacteristics. A sample obtained by selenizing a precursor sputteredat 400° C., for instance, was bent through 180 degrees at 1 cm radiuswithout cracking at room temperature.

Sputtering may occur at a variety of pressures such as normally are usedfor sputter deposition of metals. Generally, if the pressure is too low,it may be difficult to light or sustain a plasma. If too high, thesputtered species may lose too much kinetic energy. This does notpreclude the use of high pressures for sputtering, but the results maybe non-optimal. High pressures also can be difficult to implement withvacuum equipment when it is desirable to obtain low background pressuresof water vapor, nitrogen, and other contaminants. Desirably, thepressure is selected so that, when coupled with the target-to-substratedistance used, interactions among the sputtered species are minimized asmuch as is practical, and so that kinetic energy of the species ispreserved in large part. One exemplary pressure would be in a range fromabout 1 mTorr to about 50 mTorr. More preferably, the pressure would bewithin a standard magnetron sputter range of about 3 mTorr to about 20mTorr. In some instances, such as when using confocal targets, shieldingand gas injection can be used to minimize the risk that sputteredspecies could contaminate other target(s).

The film precursor is subjected to one or more chalcogenizationtreatments. Chalcogenization generally refers to exposing the filmprecursor to at least one chalcogen source under conditions to cause thechalcogen content of the film to increase. For instance, if the filmprecursor includes a sub-stoichiometric amount of chalcogen(s),chalcogenization can be carried out so that the chalcogen content isincreased to be substantially at the stoichiometric amount, or even inexcess relative to the stoichiometric amount. Chalcogenization generallyhelps convert the CIGS precursor film into a photoactive chalcopyritefilm with substantially isotropic phase formation throughout the film.

The one or more chalcogen sources used for chalcogenization may includeone or more gases, solids, liquids, gels, combinations of these, and thelike. Exemplary gas phase sources of chalcogen include H₂S, H₂Se,combinations of these, and the like. In some illustrative embodiments,the gas source is generated as vapor via evaporation from solid orliquid material and is present in an overpressure in order to facilitatemass transfer of chalcogen into the film. Exemplary solid phasechalcogen sources include S or Se, combinations of these, and the like.In some illustrative embodiments, a solid cap of one or morechalcogen-containing species is provided in intimate contact with thesurface of the precursor film to carry out chalcogenization. In otherillustrative embodiments, chalcogenization may be carried out byexposing the precursor film to both gas phase chalcogen source(s) aswell as one or more solid caps.

Chalcogenization often occurs at temperature(s) that are sufficientlyhigh to achieve the desired chalcogenization in a reasonable time periodwithout undue risk of degrading components of the workpiece(s) beingtreated. In exemplary embodiments involving selenization and/orsulfurization, a chalcogenization treatment may occur at one or moretemperatures in the range from about 300° C. to about 650° C.,preferably about 500 to about 575 for a time period of about 10 secondsto about 2 hours, preferably 10 mins to about 20 mins. Additional timemay be used to ramp temperature up and down according to a desired rampprofile. Ranges of ramp speeds commonly used include 30 C/min to 350C/min or higher. The chalcogen source may be applied and removed at anytime(s) during such ramps as desired. The chalcogen supply may bemaintained as the sample cools down to approximately 200° C. to about400° C. to help avoid loss of chalcogen from the near surface of thefilm.

An optional crystallization step may be carried out in order to convertthe precursor film to the final desired crystal form in the event thefilm is not yet in such form. For instance, if sputtering occurs at atemperature below about 500° C., crystallization may be required toconvert the precursor film into a desired chalcopyrite structure withgrains leading to high electronic quality. On the other hand, aboveabout 500° C., the precursor film may already be in the desiredcrystalline form and an anneal itself may not be needed. Even in suchinstances, may be desirable to anneal in any event to improvecrystalline characteristics. Chalcogenization is performed even ifannealing is not needed or desirable.

The optional crystallization step can be carried out using any desiredmethodology. Annealing the film precursor at suitable temperature(s) fora suitable time period at a suitable pressure is one convenient way toaccomplish annealing. Because chalcogenide is already distributedthroughout at least a portion of the film, the temperature and/or timeto achieve desired crystallization may be less than if the precursor ischalcogen-free and is formed using conventional techniques. As generalguidelines, annealing may occur at a temperature in the range from about400 C to about 650 C for a time period in the range from about 10 minsto about 60 mins. Desirably, annealing may occur in a suitableprotected, non-oxidizing, dry environment such as a vacuum. The optionalannealing step may occur prior to subsequent chalcogenization.Alternatively, annealing may occur at least in part or wholly at thesame time that chalcogenization is carried out.

One exemplary method 10 for incorporating CIGS material into aphotovoltaic device using principles of present invention isschematically shown in FIG. 1. Method 10 is representative ofembodiments in which a single target 21 is used to form a single layerprecursor film 23. In step 12, a representative substrate 20 is providedonto which precursor CIGS film 23 will be grown in step 14. Substrate 20generally refers to the body on which the CIGS precursor film 23 isformed and often incorporates multiple layers. In the illustrativeembodiment, substrate 20 generally includes support 22 and backsideelectrical contact region 24. Support 22 may be rigid or flexible, butdesirably is flexible in those embodiments in which the resultantphotovoltaic device 25 may be used in combination with non-flatsurfaces. The backside electrical contact region 24 provides aconvenient way to electrically couple the resultant device 25 toexternal circuitry.

In step 14, a single target 21 is used to sputter CIGS precursor film 23onto the substrate 20. The target 21 as shown generally includes Cu, In,Ga, and Se according to the proportions CuIn_((1-x))Ga_(x)Se_(y),wherein x is in the range from 0 to 1 and y is in the range from about0.01 to about 2, preferably about 0.1 to about 1.95, more preferablyabout 0.2 to about 1.85. These targets may also include Na, O, S, Al, orLi if desired.

In step 16, the precursor film 23 is annealed to increase thecrystalline content of the film 23, preferably so that at leastsubstantially the entire film 23 has the desired crystalline structure,grain size, or intermediary step. Next in step 18, the precursor film 23is subjected to a chalcogenization treatment in order to convert film 23into a CIGS photoabsorber layer hereinafter also referred to as absorberregion 23. Steps 16 and 18 may occur at the same time. One method ofchalcogenization would further include the evaporation of a Se cap ontop of region 23 prior to the application of step 16, or between step 16and step 18.

In step 19, additional layers are added to complete photovoltaic device25 in accordance with conventional practices now or hereafter developed.Each of these additional regions is shown as a single integral layer,but each independently can be a single integral layer as illustrated orcan be formed from one or more layers.

A heterojunction may be formed between the absorber region 23 and thetransparent conductive layer 30. The heterojunction is buffered bybuffer region 28. Buffer region 28 generally comprises an n-typesemiconductor material with a suitable band gap to help form a p-njunction proximal to the interface 32 between the absorber region 23 andthe buffer region 28. An optional window region 26 also may be present.Optional window region 26 can help to protect against shunts. Windowregion 26 also may protect buffer region 28 during subsequent depositionof the transparent conductive layer 30. Each of these regions is shownas a single integral layer, but can be a single integral layer asillustrated or can be formed from one or more layers.

One or more electrical conductors are incorporated into the device 25for the collection of current generated by the absorber region 23. Awide range of electrical conductors may be used. Generally, electricalconductors are included on both the backside and light incident side ofthe absorber region 23 in order to complete the desired electriccircuit. On the backside, for example, backside electrical contactregion 24 provides a backside electrical contact in representativeembodiments. On the light incident side of absorber region 23 inrepresentative embodiments, device 25 incorporates a transparentconductive layer 30 and collection grid 36. Optionally an electricallyconductive ribbon (not shown) may also be used to electrically couplecollection grid 36 to external electrical connections.

A protective barrier system 40 is provided. The protective barriersystem 40 is positioned over the electronic grid 36 and helps to isolateand protect the device 10 from the environment, including protectionagainst water degradation. The barrier system 40 optionally also mayincorporate elastomeric features that help to reduce the risk of damageto device 10 due to delamination stresses, such as might be caused bythermal cycling and or localized stress such as might be caused byimpact from hail and or localized point load from the weight of aninstaller or dropped tools during installation.

Additional details and fabrication strategies for making layers andfeatures 22, 23, 24, 26, 28, 30, 36, and 40 are described in U.S.Provisional Patent Application Ser. No. 61/258,416, filed Nov. 5, 2009,by Bryden et al., entitled MANUFACTURE OF N-TYPE CHALCOGENIDECOMPOSTIONS AND THEIR USES IN PHOTOVOLTAIC DEVICES; U.S. ProvisionalPatent Application Ser. No. 61/294,878, filed Jan. 14, 2010, by Elowe etal., entitled MOISTURE RESISTANT PHOTOVOLTAIC DEVICES WITH EXPOSEDCONDUCTIVE GRID; U.S. Provisional Patent Application Ser. No.61/292,646, filed Jan. 6, 2010, by Popa et al., entitled MOISTURERESISTANT PHOTOVOLTAIC DEVICES WITH ELASTOMERIC, POLYSILOXANE PROTECTIONLAYER; U.S. Provisional Patent Application Ser. No. 61/302,667, filedFeb. 9, 2010, by Feist et al., entitled PHOTOVOLTAIC DEVICE WITHTRANSPARENT, CONDUCTIVE BARRIER LAYER; and U.S. Provisional PatentApplication Ser. No. 61/302,687, filed Feb. 9, 2010, by DeGroot et al.,entitled MOISTURE RESISTANT PHOTOVOLTAIC DEVICES WITH IMPROVED ADHESIONOF BARRIER FILM, each of which is independently incorporated herein byreference for all purposes in its respective entirety.

FIG. 2 schematically illustrates an alternative embodiment of a method50 in which multiple targets are used to sputter a single layerprecursor film 64 onto a substrate 52. In step 51, a substrate 52 isprovided. As in FIG. 1, an illustrative substrate construction is shownincluding support 54 and backside electrical contact 56. In Step 58,confocal targets 60 and 62 are used to sputter CIGS film precursor layer64 onto substrate 52. Target 60 has the composition Cu_(x)Se_(y),wherein x is approximately 2 and y is approximately 1. Otherchalcogen-containing targets could be used in place of target 60 and/orin combination with target 60. For example, an alternativechalcogen-containing target could be the Cu, In, Ga, and Se targetdescribed in FIG. 1. As another alternative, some or all of the Se intarget 60 could be substituted with S and/or Te. Target 62 generally ischalcogen-free and incorporates Cu, In, and Ga according to the formulaCuIn_(p)Ga_((1-p)), wherein p desirably is in the range from about 0.5to about 1. After the precursor film 64 is formed, additional steps (notshown) can be performed similar to those in FIG. 1 to accomplishannealing, chalcogenization, and completion of the resultantphotovoltaic device. Other Cu, In, or Ga-containing targets could beused in place of target 62 and/or in combination with target 62.

FIG. 3 schematically illustrates another embodiment of a method 70 inwhich multiple targets are used to sputter a multilayer precursor film74 onto a substrate 72. In step 71, confocal targets 76 and 78 are usedto sputter a first precursor layer 80 onto substrate 72. For purposes ofillustration, target 76 incorporates chalcogen, while target 78 ischalcogen-free. Specifically, target 76 incorporates Cu and Se in thesame proportions as are used in FIG. 2. Other chalcogen-containingtargets could be used in place of target 76 and/or in combination withtarget 76. For example, an alternative chalcogen-containing target couldbe the Cu, In, Ga, and Se target described in FIG. 1. As anotheralternative, some or all of the Se in target 76 could be substitutedwith S and/or Te. Target 78 generally is chalcogen-free and incorporatesCu, In, and Ga according to the formula CuIn_(p)Ga_((1-p)), wherein pdesirably is in the range from about 0.5 to about 1. Other Cu, In, orGa-containing targets could be used in place of target 78 and/or incombination with target 78.

In step 73, confocal targets 82 and 83 are used to sputter achalcogen-free layer 84 over layer 80. In a representative embodiment,target 83 contains Cu. Target 82 contains In. Sputtering can occur at afaster rate in step 73, because both targets 82 and 83 arechalcogen-free. Alternatively, both confocal targets 82 and 83 cancontain Cu and In, or Cu, In, and Ga, at different ratios, such that paccording to the formula CuIn_(p)Ga_((1-p)), is in the range from about0.5 to about 1, and the ratio of (In+Ga)/Cu is between 1.2 and 1.8.

In step 85, confocal targets 86 and 88 are used to depositchalcogen-containing layer 90 to complete precursor film 74. Target 86can be a chalcogen-containing target such as any of the Se-containingtargets described above with respect to this FIG. 3 or FIGS. 1 and 2.Target 88 is a chalcogen-free target containing Cu, In, and Ga asdiscussed above. After the precursor film 74 is formed, additional steps(not shown) can be performed similar to those in FIG. 1 to accomplishannealing, chalcogenization, and completion of the resultantphotovoltaic device.

The present invention will now be further described with reference tothe following illustrative examples.

Comparative Example A Chalcogenization of Sputtered Precursor Wherein NoChalcogen is Included in the Precursor

Comparative CIGS film A is prepared by post-selenizing a precursor filmdeposited using a CIG alloy target, where the atomic ratio of[In+Ga]/Cu=1.2. No Se or other chalcogen is incorporated into theprecursor film. Film A is deposited onto a 5×5″ piece of stainless steelcoated with Nb (150 nm) and Mo (350 nm) prior to the precursordeposition. A 100 diameter CIG target is sputtered at 120 W for 32minutes with an Ar pressure of 5 mTorr to deposit a Se-free precursorfilm. Post-selenization is completed by evaporating a Se cap onto theprecursor film, at room temperature, of 25 times the moles of Cu in theprecursor film. The Se-capped precursor film is heated at approximately30° C./min up to 550° C., holding at that temperature for 20 minutes,and allowing the sample to cool. The final film is approximately twomicrons thick.

Comparative Sample A shows large voids primarily at the Mo-CIGSinterface. Some small voids, approximately 0.1 microns to 0.5 micronswide, are located within the CIGS film itself. These “internal” voidsmake up less than approximately 10% of the total void area as measuredby polished SEM cross section. The “large interface” voids in thismaterial are approximately 0.2 to 2.0 microns long in the directionparallel to the substrate plane, and between approximately 0.2 to 1.0microns in the direction perpendicular to the substrate plane. The largeinterface voids are located primarily at the Mo-CIGS interface, makingup approximately 90% of the total void volume. These micron-sized voidsat the Mo-CIGS interface contribute to very poor adhesion of the CIGS tothe underlying substrate. If a cross-sectional area is defined by the Mosurface at the bottom and CIGS surface at the top, following themeasurement technique described herein where multiple, polishedcross-section samples have a cumulative length of at least 20 micronsare evaluated, the void content is approximately 15% of the film areacomputed as an average of the samples.

The Ga profile of Comparative A is measured by scanning XTEM with EDSlinescan. The Ga is observed to completely segregate toward the back ofthe film such that there is 0.0 atomic percent Ga detected in the top0.5 microns of the CIGS film. The level of Ga at the back side of thefilm reached 20.4 atomic percent. A working photovoltaic device was madewith this baseline material. The Voc (open circuit voltage) for a cellwith 7% efficiency was 365 mV. Quantum efficiency measurements of thisdevice yield a bandgap estimate of 0.98, confirming the complete lack ofany Ga benefit in the photoabsorbing area of the CIGS film.

Example 1 Preparation of CIGS Film Embodiment of the Present Invention

A CIGS film precursor is produced with a relatively high butsubstoichiometric level of Se throughout the precursor film. A “CIGS”sputter target (commercially available) containing the atomic ratios ofIn+Ga/Cu of 1.2, Ga/In of 0.3, and Se/(Cu+In+Ga)=2, 2″ in diameter, isused to sputter the precursor film. The method used is pulsed DC (directcurrent) sputtering, with a pulse frequency of 330 kHz and an off timeof 1.2 μs. The substrate is located 8 cm away from the target. Thesubstrate is a piece of soda-lime glass coated with a 800 nm thick layerof Mo. The substrate was not intentionally heated during sputtering, butthe substrate temperature during growth is estimated to be approximately60° C. throughout the growth of the precursor film. The base pressure ofthe system is under 5*10⁻⁷ Ton; the sputtering pressure is 3.5 mTorrusing ultra high purity Argon gas. The target is sputtered at a power of100 W for 2.5 hours. The precursor, before selenization, has a Se/Cuatomic ratio of approximately 1.89, and a Se/(In+Ga+Cu) atomic ratio of0.92. The precursor has a uniform Ga distribution throughout theprecursor film.

Post-selenization is completed by evaporating a Se cap onto theprecursor film at room temperature. The cap includes 25 times the molesof Cu in the precursor film. The Se-capped precursor film is heated atapproximately 30 C/min up to 550° C., holding at that temperature for 20minutes, and allowing the sample to cool.

The post-selenized film composition of Ga at the near surface andthroughout the film is measured by scanning XTEM with EDS. The Gacomposition is 7 atomic percent throughout the film, without variation(to within the measurement capability of scanning XTEM with EDS,approximately +/−0.1 atomic percent.) The adhesion of the film isexcellent upon cutting and handling. The voids in the post-selenizedfilm are very different than for the Comparative Sample A. The voids ofthe present Example are much smaller on average, with a size range of afew nm to approximately 0.1 microns. The voids are much more evenlydistributed throughout the film, including at the Mo-CIGS interface, andthroughout the CIGS film. The total void content is similar, atapproximately 13%. The percentage of these voids located at the Mo-CIGSinterface is estimated to be approximately 15%.

Example 2 Preparation of CIGS Film Embodiment of the Present Invention

A CIGS film precursor is prepared with a relatively low,substoichiometric level of Se throughout the film. This film precursoris deposited with both a CIGS alloy target (Cu, In, Ga, and Se includedin the target) similar to the one used in Example 1 except that thediameter is 100 mm, and a CIG alloy target (Cu and In and Ga included inthe target) similar to the one used for comparative example A. The twotargets are sputtered at the same time in a confocal arrangement, bothat 100 W for 30 minutes at room temperature. As the CIGS target sputtersmuch more slowly than the CIG target, this results in a bulk film with arelatively low concentration of Se throughout. In this case, the targetswere 100 mm in diameter, and the substrate is a 5×5″ piece of stainlesssteel coated with Nb (150 nm) and Mo (350 nm) prior to the precursordeposition. The Ar pressure during sputtering was 5 mTorr. All otherconditions are similar to those in Example 1. The bulk composition ofthe precursor before post-selenization is measured via ICP. The totalSe/Cu atomic ratio is 0.7, and the Se/(In+Ga+Cu) atomic ratio is 0.33.The precursor is then subjected to a similar post-selenization treatmentas described in Example 1.

The film yields a working photovoltaic device. The final photovoltaiccell, including the post-selenized CIGS film, is analyzed by scanningXTEM with EDS. A reduction in the number of “Mo-CIGS interface” voids isseen. There exists a region between the Mo-CIGS interface and the bulkCIGS film that includes several very small voids as seen by scanningXTEM with EDS. This may indicate that a non-optimal post-selenizationprocess occurs for this film, and further improvement is likely. Thevoid size is approximately 0.2 microns across for the largest voidsseen. Total void volume is approximately 9%. Approximately 50% of thevoids are at the Mo-CIGS interface, and the remainder within the bottommicron of the CIGS bulk film. Scanning XTEM with EDS shows significant,but not total segregation of Ga toward the back-side of the film suchthat Ga is detected in the top 0.5 microns of the CIGS film at a levelof between 0.3 atomic percent to 0.9 atomic percent.

A photovoltaic device made using with this film with 7% efficiency had aVoc of 350 mV, and showed a bandgap of approximately 1.02 eV as measuredby quantum efficiency, which is slightly improved over that ofcomparative example A.

Example 3 Preparation of CIGS Film Embodiment of the Present Invention

A precursor film is prepared that has a multilayer format to demonstratethe back-side layer benefit of an Se-containing precursor. The precursoris deposited by first using a CIGS target, (the same CIGS target usedfor example 2), to deposit a layer for 28% of the total deposition time,with the reminder of the film deposited from a metallic CIG alloytarget,(the same CIG target used for example 2). The total time ofdeposition is 46 minutes, and the power on both targets is 100 W. Thesedepositions are performed at room temperature, using a gas pressure of 5mTorr. The two sputter sources are arranged in a confocal geometry, andsimilar pulsed DC sputtering conditions and target-to-substrateconditions are used as Examples 1 and 2. The substrate is also similarto the substrate of example 2, being a 5×5″ piece of stainless steel,coated with Nb (150 nm) and Mo (350 nm) prior to the precursordeposition. The precursor of the present example is then subjected to asimilar post-selenization treatment as described for Examples 1 and 2.

The bulk composition of the precursor film of this example beforepost-selenization is measured via ICP. The total Se/Cu atomic ratio is0.28, and the Se/(In+Ga+Cu) atomic ratio is 0.11. The film yielded aworking photovoltaic device. The final photovoltaic cell, including thepost-selenized CIGS film, is evaluated by scanning XTEM with EDS. Adramatic reduction in the number of “large Mo-CIGS interface” voids isseen as compared to the Comparative Sample A. A thin layer of CIGS alsoexists, visible by XSEM and XTEM, attached directly to the Mo film atover 90% of the measured interface length. The voids remained “long” inthe direction parallel to the substrate plane (approximately twomicrons), but were shorter in the dimension perpendicular to thesubstrate (approximately 0.8 microns to less than 0.1 microns). Theadhesion of the film is observed by cutting and handling to bedramatically improved over the Comparative Sample A. Scanning XTEM withEDS shows total segregation of Ga toward the back-side of the film, witha level of Ga in the top 0.5 microns of the film of 0.0%. However, inthe back 0.3 microns of precursor, where the CIGS (and hence Se) isoriginally deposited, there is a clear enhancement of the Gaconcentration to approximately 16-20 atomic percent. A sudden increaseof Ga content begins at the interface between the original interface ofCIG- and CIGS-precursor material and continues throughout the portioncorresponding to the CIGS precursor material. A working photovoltaicdevice is made with this material, and the Voc for a cell withapproximately 7% efficiency is 329 mV and does not demonstrate a bandgapimprovement via quantum efficiency measurements.

Example 4 Preparation of CIGS Film Embodiment of the Present Invention

A precursor film is deposited using a metallic CIG alloy target, similarto the one used for examples 2 and 3, with a small concentration of H₂Segas as the Se source added to the Ar sputter gas. All other sputter andpost-selenization conditions are similar to those described in Example#2. The concentration of H₂Se gas is approximately 5% of the Ar asmeasured by flow ratio through appropriately calibrated mass flowcontrollers. The film is deposited for 35 minutes at room temperature.Measurements were performed using similar techniques as the otherexamples. The bulk composition of the precursor before post-selenizationis measured via ICP. The total Se/Cu atomic ratio is 1, and theSe/(In+Ga+Cu) atomic ratio is 0.48.

Upon post-selenization, this film demonstrates an altered void structurecontaining a network of smaller voids both at the Mo-CIGS interface andwithin the CIGS itself as measured with scanning XTEM with EDS.Approximately 50% of the voids were contained within the CIGS itself,and approximately 50% at the CIGS-Mo interface. Void size is smallerthan those observed in the Comparative Sample A. The average void sizeis 0.25 microns parallel to the substrate direction, and 0.1 micronsperpendicular to the substrate direction. The adhesion of this film wascomparable to that of the Comparative Sample A. The Ga distribution inthe final film is significantly improved toward the front side of thefilm. The Ga distribution is measured by scanning XTEM with EDS to beapproximately 5.0 atomic percent within the top 0.5 microns of the film.The Ga distribution steadily rises toward the back of the film to alevel of approximately 12 atomic percent.

This film is made into a photovoltaic device that demonstrates improvedquantum efficiency and Voc (open circuit voltage) characteristics overthe Comparative Sample A. The quantum efficiency demonstrated asignificant shift towards lower wavelengths, indicating a bandgap ofapproximately 1.12. This is improved over that of the Comparative SampleA that had quantum efficiency measurements indicating a bandgap of 0.98eV. The Voc for a cell with 8% efficiency is 503 mV compared to 365 mVfor the Comparative Sample A. Both of these improvements support theexistence of a significant amount of Ga within the photoabsorbing regionof the CIGS film.

Other embodiments of this invention will be apparent to those skilled inthe art upon consideration of this specification or from practice of theinvention disclosed herein. Various omissions, modifications, andchanges to the principles and embodiments described herein may be madeby one skilled in the art without departing from the true scope andspirit of the invention which is indicated by the following claims.

What is claimed is:
 1. A photovoltaic device comprising a chalcogen-containing photoabsorbing film made by a process comprising the steps of: (a) sputtering at least one target in the presence of one or more chalcogens comprising Se to form a Se-containing precursor of the chalcogen-containing photoabsorber wherein the Se-containing precursor includes a sub-stoichiometric amount of the one or more chalcogens comprising Se throughout the precursor or in selected areas of the precursor; and (b) while the precursor comprises Se throughout the precursor or in the selected areas of the precursor, subjecting the Se-containing precursor to a thermal treatment in the presence of one or more chalcogens comprising Se that selenizes the precursor to form the selenium-containing photoabsorbing composition.
 2. A photovoltaic device, comprising a chalcogen-containing photoabsorbing film having a top surface and a bottom surface, said photoabsorbing film comprising: a void content of 0.5 to 50%; and 0.1 to 60 atomic percent Ga at least in a first boundary region proximal to the bottom surface of the photoabsorbing film and/or a second boundary region proximal _(t)o the top surface of the photoabsorbing film, wherein the atomic percent Ga is based on _(t)h_(e) total composition of the corresponding boundary region.
 3. The photovoltaic device of claim 2, wherein the Ga is included in the first and second boundary regions and wherein at least one additional region is interposed between the first and second boundary regions, wherein the additional region includes a reduced Ga content relative to the first and second boundary regions.
 4. The photovoltaic device of claim 2, wherein the photoabsorbing film incorporates ingredients comprising Cu, In, Ga, and Se.
 5. The photovoltaic device of claim 2, wherein at least one of the first and/or second boundary regions has a thickness in the range from about 10 nm to about 300 nm and the at least one boundary region comprises from about 2 to about 10 atomic percent Ga based on the total composition of the at least one boundary region.
 6. The photovoltaic device of claim 2, further comprising an electrical contact region on the bottom surface of the chalcogen-containing photoabsorbing film and having an interface therebetween.
 7. The photovoltaic device of claim 2, wherein at least more than 10% of the total void area includes voids that contact inner surfaces of the photoabsorbing film only and do not contact the interface between the chalcogen-containing photoabsorbing film and the electrical contact region.
 8. The photovoltaic device of claim 2, wherein at least one chalcogen is incorporated into at least one of the first and second boundary regions.
 9. The photovoltaic device of claim 2, wherein at least one chalcogen is incorporated into each of the first and second boundary regions.
 10. The photovoltaic device of claim 9, wherein the chalcogen is Se.
 11. The photovoltaic device of claim 2, wherein the void content comprises voids having an average size of from 0.1 microns to 2 microns.
 12. A precursor film of a chalcogen-containing photoabsorbing material, said precursor comprising from about 0.1 to about 60 atomic percent Ga selectively co-incorporated into one or more portions of the precursor with at least one chalcogen based upon the total composition of said portion; and wherein the precursor includes a sub-stoichiometric amount of a chalcogen based upon the overall composition of the precursor film.
 13. The photovoltaic device of claim 12, wherein the precursor film is at least partially amorphous.
 14. The photovoltaic device of claim 12, wherein the chalcogen comprises Se.
 15. The precursor film of claim 12, wherein the precursor comprises at least two metals selected from Cu, In, and Ga, and the atomic ratio of chalcogen to the cumulative amount of metals in the precursor is in the range from about 0.01 to 0.8.
 16. The precursor film of claim 12, wherein the precursor film has a top and a bottom, and wherein the precursor comprises a first region proximal to the bottom, a second region proximal to the top, and at least one region interposed between the first and second regions, wherein at least one chalcogen comprising Se is selectively incorporated into at least one of the first and second regions and is not included in at least one of the interposed regions.
 17. The precursor film of claim 16, wherein Ga is selectively co-incorporated into at least one of the regions that also include at least one chalcogen comprising Se. 